1. Field of the Invention
The present invention generally relates to systems and methods for measuring one or more characteristics of patterned features on a specimen. Certain embodiments relate to a system that includes an optical subsystem configured to acquire measurements of light scattered from patterned features on a specimen at multiple angles of incidence, multiple azimuthal angles, and multiple wavelengths simultaneously.
2. Description of the Related Art
The following description and examples are not admitted to be prior art by virtue of their inclusion in this section.
Metrology processes are used at various steps during a semiconductor manufacturing process to monitor and control the process. Metrology processes are different than inspection processes in that, unlike inspection processes in which defects are detected on a wafer, metrology processes are used to measure one or more characteristics of the wafer that cannot be determined from currently used inspection tools. For example, metrology processes are used to measure one or more characteristics of a wafer such as a dimension (e.g., line width, thickness, etc.) of patterned features formed on the wafer during a process such that the performance of the process can be determined from the one or more characteristics. In addition, if the one or more characteristics of the wafer are unacceptable (e.g., out of a predetermined range for the characteristic(s)), the measurements of the one or more characteristics of the wafer may be used to alter one or more parameters of the process such that additional wafers manufactured by the process have acceptable characteristic(s).
Generally, the sensitivity of scatterometric metrology systems to certain variables of the specimen under test is closely related to the angle of incidence (AOI, which will be represented by the symbol θ in following description) and azimuthal angle (represented by the symbol φ) of the beam falling onto the specimen. One typical example of a metrology system measurement is the measurement of the thickness of a thin silicon oxide (SiO2, which is commonly referred to as “oxide”) film on a silicon substrate. The sensitivity of the measurement to the oxide thickness is maximized when the AOI is set close to the Brewster angle of the silicon substrate. This AOI leads to the design of spectroscopic ellipsometers (SE) with an AOI at approximately 71 degrees, which has been an optimized system setup for most film measurement applications. In addition, because most materials in semiconductor manufacturing are optically isotropic, azimuthal angle is largely ignored in film thickness measurements. For critical dimension (CD) measurements, however, the above AOI is not necessarily the best configuration as far as the sensitivity to certain CD characteristics is concerned.
There are some ways for varying the AOI in spectroscopic systems. Nevertheless, none of them has been applied for CD measurements. The simplest and most straight-forward way for varying AOI in spectroscopic systems is to change both the direction of the incident beam and the collection beam simultaneously. Tools having such capability include the VASE® (Variable Angle Spectroscopic Ellipsometry) series of systems that are commercially available from J. A. Woollam Co., Inc., Lincoln, Nebr. A modified version of this “2-θ” scanning scheme is described in D. M. Byrne and D. L. MacFarlane, “Angular scanning mechanism for ellipsometers,” Applied Optics, 30(31), 4471-4473, (1991), which is incorporated by reference as if fully set forth herein, in which two flat turning mirrors are used to change the angle of the incident beam and the reflection beam simultaneously.
The most significant drawback of this scheme, particularly for CD measurements, is the requirement of two synchronized rotations. As the technology node continuously decreases to 45 nm and lower, the performance requirements for metrology systems are continuously raised. To meet leading-edge specifications of precision, accuracy, and tool-to-tool matching for CD measurements, it is critical to calibrate system parameters substantially accurately and to maintain a relatively high level of stability in these parameters during measurements. AOI is one of these critical system parameters that needs to be exactly calibrated and stabilized. The AOI of the above VASE design, as a result of two rotating elements, is difficult to calibrate and maintain. This difficulty is a major hurdle preventing the above VASE design from being adopted in semiconductor production lines, and as a result, the main applications of systems based on this scheme are mostly research applications. In addition, this scheme does not have the capability to vary azimuthal angles.
One attempt to acquire AOI- and azimuthal angle-resolved information simultaneously has been developed by various companies, which involves using a focusing mirror or lens with a relatively large numerical aperture (NA). U.S. Pat. No. 5,889,593 to Bareket, which is incorporated by reference as if fully set forth herein, illustrates the basic principle of this scheme for reflectometry instead of ellipsometry. In this type of scheme, a relatively large NA objective (e.g., an objective having an NA of at least 0.5, and preferably up to 0.95), either a mirror or lens (such as a microscope objective), focuses an illumination beam onto the specimen. Because of the large NA, the incident beam includes many different AOIs (in the case of an NA of 0.95, AOIs ranging from 0° to 70°). To resolve the AOIs, an imaging array is used as a light detector. Signals corresponding to different AOIs and azimuthal angles fall onto different pixels of the imaging array and are recorded. Similar schemes are also disclosed in U.S. Pat. Nos. 5,596,406 to Rosencwaig et al., 5,703,686 to Leroux, 5,880,845 to Leroux, 6,556,284 to Leroux, and 6,654,131 to Opsal et al., as well J. Petit, et al., “A new analysis strategy for CD metrology using rapid photo goniometry method,” Proceedings of SPIE 5375, 210 (2004), all of which are incorporated by reference as if fully set forth herein.
One major disadvantage of this type of angle-resolved design is the difficulty to perform spectroscopic measurements as well, which involves a three-dimensional data cube of AOI, azimuthal angle, and wavelength. Recording this data simultaneously is beyond the capability of a two-dimensional imaging array. To resolve the angular and spectral information, some complex wavelength multiplexing-demultiplexing is needed, which may either not work efficiently or may increase the complexity and reduce the reliability of the system.
For example, in a monochromatic measurement system, the system configuration is relatively simple and straightforward. If AOI-resolved measurements are desired, a linear imaging array can be placed in the direction parallel to the plane of incidence. In this manner, signals corresponding to different AOIs are mapped to different pixels in the imaging array. If azimuthal angle-resolved measurements are desired, an arc-shaped mask can be placed in the plane of an exit aperture to just pick up reflection signals with a given AOI but various azimuthal angles as described by J. Petit, et al. in the article incorporated by reference above. In this type of azimuthal angle-resolved system, a two-dimensional detector array is used even though only a few pixels in the two-dimensional array corresponding to the arc-shaped mask are used for the measurements.
However, when spectroscopic measurements are desired, some complex means is needed to resolve the spectral information. One method for resolving the spectral information is to record signals of different wavelengths at different times such as by using a monochromator or filter wheel to allow light of only one wavelength to enter the system at a time. The obvious drawback of this method is that the throughput of the measurement system is reduced, and as the number of wavelengths at which measurements are performed increases, the throughput of the system decreases.
In the above disclosed methods, the chief rays of illumination and reflection are at normal incidence. As such, the largest AOI is limited by the NA of the focusing optics. As a consequence, careful correction of the aberrations of the focusing optical systems are needed. In addition, because of the large NA, the illumination spot is relatively small, typically in the range of a few μm. However, scatterometry is based on model-based analysis, which in general is performed utilizing rigorous coupled-wave analysis (RCWA), in which illumination of a periodic patterned structure is required. According to the International Technology Roadmap for Semiconductors (ITRS), the most challenging layers have relatively small CDs with relatively large pitches (e.g., typically CDs of less than 50 nm and pitches in the range of 800 nm). When the illumination spot is too small, the periodicity of the patterned features under measurement may not hold anymore.
U.S. Pat. No. 5,166,752 to Spainer et al. and U.S. Patent Application Publication No. 2004/0196460 A1 by Dobschal et al., which are incorporated by reference as if fully set forth herein, disclose systems that are configured to direct light to a specimen at an oblique AOI. Additionally, these two systems include a two-dimensional detector in combination with a dispersion element (such as a grating or a prism) for AOI-resolved spectroscopic measurements. In such systems, columns of the detector correspond to signals with a fixed wavelength but various AOIs, and rows of the detector correspond to signals with a fixed AOI but various wavelengths. Therefore, a pixel in a specific row and column of the detector will theoretically pick up only the signal for a specific wavelength and a specific AOI.
A significant problem related to such a system configuration is shown in FIG. 1 of U.S. Pat. Nos. 5,596,406 to Rosencwaig et al. and 6,654,131 to Opsal et al., which are incorporated by reference above, and described in the corresponding description of this figure. Briefly, because of the finite sizes of the illumination and reflection beams, a pixel in a specific row and column of the detector will not only record the signal corresponding to the wavelength and AOI specified by the column number and row number. Instead, signals from adjacent pixels corresponding to different wavelengths and AOIs may also fall onto this pixel thereby degrading both the spectral and angular resolutions.
To overcome the above problem, Rosencwaig et al. and Opsal et al. suggest using a rectangular aperture that is elongated in one direction and placed in front of the dispersion element. In this manner, only signals corresponding to the azimuthal angle parallel to the long-direction of the aperture can be picked up thereby reducing the three-dimensional data cube (AOI, azimuthal angle, wavelength) into a two-dimensional “data plane” (AOI, wavelength). As a result, this system configuration effectively eliminates the resolution degradation problem in the systems disclosed by Spainer et al. Nevertheless, this configuration creates its own problem, namely, that the light power utilization is much lower because only a relatively small portion of the light corresponding to a given azimuthal angle is collected.
Furthermore, it is more difficult to facilitate azimuthal angle-resolved spectroscopic measurements than the AOI-resolved spectroscopic measurements disclosed above, for two reasons. First, when the chief ray is at normal incidence, the exit aperture is mapped to an entire reflection light cone corresponding to azimuthal angles in the range of 0 degrees to 360 degrees. On the other hand, under oblique incidence, the range of azimuthal angles is significantly reduced, which is defined by the NA of the collection optics. However, for CD measurements, it may be desirable to perform measurements at azimuthal angles in a relatively large range, which may vary from 0 degrees to 90 degrees. Obviously, the systems disclosed by Spainer et al. and Dobschal et al. are insufficient for this purpose.
In addition, at both normal and oblique incidence, to perform azimuthal angle-resolved measurements, an arc in the exit aperture (such as that defined by an arc-shaped mask described in the paper by J. Petit et al.) has to be mapped to a line in the direction parallel to one of the axes in the two-dimensional detector array. Optically, such mapping is complex and expensive to implement.
Accordingly, it may be advantageous to develop systems and methods for measuring one or more characteristics of patterned features on a specimen that can acquire measurements of light scattered from the patterned features on the specimen at multiple AOIs, multiple azimuthal angles, and multiple wavelengths simultaneously while eliminating one or more of the disadvantages of the methods and systems described above.